Chromate conversion coating compositions containing prusside accelerator

ABSTRACT

A composition for applying a chromate conversion coating on aluminum comprised of hexavalent chromium, fluoride, and a prusside as an accelerator selected from the group consisting of Na2( Fe(CN)5NH3) ; Na3( Fe(CN)5NH3); Na3( Fe(CN)5H20); Na2(Fe(CN)5H2O)) and Na3( Fe(CN)5NO2).

United States Patent Inventor Kurt Goltz Exton, Pa. Appl. No. 7,992 IFiled Feb. 2, 1970 Patented Oct. 26, 1971 Assignee Pennwalt CorporationPhiladelphia, Pa.

CI-IROMATE CONVERSION COATING COMPOSITIONS CONTAINING PRUSSIDEACCELERATOR 9 Claims, No Drawings Int. Cl C09d 5/08 Field of Search106/14;

[56] References Cited UNITED STATES PATENTS 2,796,371 6/1957 Ostranderet al. 148/62 2,988,465 6/1961 Newhard et al.... 148/62 3,348,97910/1967 Murphy et al. 148/62 Primary Examiner- Lorenzo B. HayesAttorneys-Stanley Litz and Carl A. l-Iechmer, Jr.

.CHROMA'IE CONVERSION COATING COMPOSITIONS CONTAINING PRUSSIDEACCELERATOR This invention relates to a composition and method forforming a chromate conversion coating on aluminum and aluminum alloys.More particularly, this invention concerns a chromate and fluoride-basedconversion coating composition for treating aluminum and aluminum alloyscontaining at least one specifically named prusside compound as anaccelerator.

The normally used method for treating aluminum and aluminum alloys toprovide long lasting corrosion protection, and/or to promote adhesionthereto of an organic finish, such as a paint or lacquer, is to apply abonded chromate conversion coating by immersing the aluminum in anaqueous solution containing chromate ion and fluoride ion and anaccelerator to accelerate the chromate conversion coating action. Thepresent invention provides an improvement over previously known chromateconversion chromate compositions, such as disclosed in U.S. Pat. No.2,796,370 describing a ferrocyanide or ferricyanide accelerator. Thepractice of this invention permits the application of suitableconversion coatings to aluminum in from about 10 to 15 percent less timethan required with the previously known compositions, using equivalentaccelerator concentrations. This time savings results in sharply reducedcosts in modern plant operations where thousands of tons of aluminum areprocessed through complex and expensive coating equipment.

The composition of this invention useful in the art of applying a bondedcoating on aluminum and aluminum alloy surfaces comprises a mixture ofhexavalent chromium, a fluorinebearing compound and a prusside selectedfrom the group consisting 9L :[F C 5 Ha] 3 CN)5.NH3l; Na;,[Fe(CN) l-l Na[Fe(CN) H O] and Na [Fe(CN) N0 in proportions capable of coating saidaluminum and alloys thereof.

The composition of this'iiiveritionin the preferred embodiment comprisesan aqueous solution having a pH within the range of about L2 to about22, more preferably 1.5 to 2.0, and containing in solution from about 1to grams per liter of solution of hexavalentchromium ion, from about 0.1gram to about 10 grams per liter of fluoride ion (plain and/orcomplexed), small amounts of optional and nonessential other materialssuch as nitrates and metal ions including iron, nickel and zinc ions andalkali metal and alkaline earth metal ions, and as an accelerator fromabout 0.1 gram to about 10 grams/liter, more preferably 0.5 to 2grams/liter, of a prusside compound selected from the group consistingof: Na [Fe(CN) s a] a[ )5 a] aI fl L-HZ il fl h H O and Na [Fe(CN) NO,].

The hexavalent chromium ion in the conversion coating composition can beprovided by chromic acid or water soluble salts thereof such as sodiumbichromate, potassium bichromate, sodium chromate and potassiumchromate. The fluoride ion can be supplied by such representativefluorine-bearing compounds as sodium fluoborate, sodium fluoride,potassium fluoride, sodium fluosilicate, potassium fluosilicate,ammonium fluosilicate, ammonium fluoride, magnesium fluoride and thelike. The pH of the solution can be adjusted, if necessary, by additionof mineral acids thereto such as nitric acid or fluosilicic acid. Theprusside compounds embodied in the compositions are known materials andmethods for their preparation may be found in the literature, e.g.,Gmelins Handbuch der Inorganischen Chemie, 8, Ausgabe, Band Eisen B.

According to the method of this invention, the aluminum; (which is firstadvantageously cleaned by conventional methods to remove organic matterresidues and metallic oxides) is treated with the aqueous-coatingsolution of the invention by immersion, brushing or spraying at atemperature within the range of about 0 C. to 50 C., preferably roomtemperature, i.e. about C. to C. As stated previously, the advantage ofthe present invention is that reduced treatment times are possible,e.g., from about three seconds to about fifteen seconds depending on thetype of operation, aresufficient using the present composition to applyadequate corrosion protective and adhesion-promoting coatings to thealuminum thus treated.

The data presented in the following examples illustrate the improvedresults from the present composition and demonstrates the impossibilityof predicting whether a specific compound will be a good coatingaccelerator. For instance, it is demonstrated that prusside compoundssimilar to those claimed herein give either comparatively poor resultsin chromate conversion coating or are almost totally inoperative.

A typical chromate conversion coating stock solution is preparedcomprising an aqueous solution containing 4.5 grams/liter chromic acid;3.9 grams/liter(Nl-l ),SiF and 1.7 grams/liter Na,SiF the pH is adjustedto 1.8 with nitric acid, and to separate portions of this solution anaccelerator com- .pound is added in the concentrations as set forthhereinbelow.

Panels of 3003 aluminum alloy are cleaned in a conventional, aqueous,alkaline, silicate-phosphate soak cleaner. The aluminum panels aredipped in the chromate conversion coating bath for 15 seconds at roomtemperature. The weight of the coatings applied to the aluminum panelsin milligrams per square foot is determined according to the NationalCoil Coaters Association Bulletin "-2. The results of the tests usingthe various accelerators are set forth in the following table.

Accelerator Coating conccntraweight,

Ex. Accelerator tion, g./l. mgJlt.

1 Control (no accelerator) 0 1.5 2 Ng4[F6(CN)o]-10H:O (0.8. 2,796,- 0. 412.

7 3 Na4[Fe(CN)a]-10H:0 0.8 21.0 4 Same as above 1. 5 -0. 0 5Naa[Fe(ON);AsO:] 1.5 8.4 6 Nas Fe(CN)iS03 1.5 7-- N8.2[FG(CN)5NO -2Hz0-0.5 1.4 8.. Na3[Fe(GN)5C0 0.7 10.4 9 Na; Fe(CN) NO- 1.5 13.1 10-NBaiF8(CN)5NOz 1.6 22.5 11 Naa[Fe(CN)5NHa 1.4 22.0 12 N82[F6(CN)5NH3 0.624.0 13 Same as above 1. 3 23. 0 14 Na3[Fc(CN)rH20] 1.5 23.8 15. Na[Fe(CN);HO] 0. 4 15. 2

\ Negligible effects.

The foregoing results show that the accelerators of the claimedcomposition (Examples 10-15) give markedly higher coating weightscompared to similar compositions containing no additive, the prior artaccelerator, or structurally similar compounds.

lclaim:

1. A composition for applying a corrosion resistant coating to aluminumand aluminum alloys comprising an aqueous solution having a pH withinthe range of about l .2 to about 2.2 containing a water-soluble chromiumcompound in which the chromium is in hexavalent form in a quantitysufficient to provide from about 1 to 10 grams per liter of solution ofhexavalent chromium ion, a fluorine-bearing compound in a quantitysufficient to provide from about 0.1 gram to about 10 grams per liter ofsolution of fluoride ion, and from about 0.1 gram to about 10 grams perliter of a prusside selected from the group consisting of Na [(Fe(CN)Nl-l Na;,(Fe(CN) ',Nl-l Na (Fe(CN) l'l 0); Na (Fe(CN) ',H 0) and Na(Fe(CN),, N0

2. A composition in accordance with claim 1 wherein the pH of thesolution is within the range of 1.5 to 2.0 and the concentration of theprusside is from 0.5 to 2 grams/liter.

3. A composition according to claim 1 wherein the prusside is Na (Fe(CNNl-l 4. A composition according to claim 1 wherein the prusside 5. Acomposition according to claim 1 wherein the prusside is Na (Fe(CN H O).

6. A composition according to claim 1 wherein the prusside is Na (Fe(CN)l-l 0).

7. A composition according to'claim 1 wherein the prusside is Na (Fe(CN)NO from about 0.1 gram to about 10 grams per liter of fluoride ion, andfrom about 0.1 gram to about 10 grams per liter of a prusside selectedfrom the group consisting of Na,(Fe(CN), 3); a( )s a); a( )a z M M M H0) and Na,(Fe(CN) NO 9. The method according to claim 8 'wherein theaqueous solution is defined by claim 2.

2. A composition in accordance with claim 1 wherein the pH of thesolution is within the range of 1.5 to 2.0 and the concentration of theprusside is from 0.5 to 2 grams/liter.
 3. A composition according toclaim 1 wherein the prusside is Na2(Fe(CN)5NH3).
 4. A compositionaccording to claim 1 wherein the prusside is Na3(Fe(CN)5NH3).
 5. Acomposition according to claim 1 wherein the prusside isNa3(Fe(CN)5H2O).
 6. A composition according to claim 1 wherein theprusside is Na2(Fe(CN)5H2O).
 7. A composition according to claim 1wherein the prusside is Na3(Fe(CN)5NO2).
 8. A method for forming abonded coating on aluminum and aluminum alloys which comprisescontacting the surface of said metal with an aqueous solution having apH within the range of about 1.2 to about 2.2 containing a water-solublechromium compound in which the chromium is in hexavalent form in aquantity sufficient to provide from about 1 to 10 grams per liter ofsolution of hexavalent chromium ion, a fluorine-bearing compound in aquantity sufficient to provide from about 0.1 gram to about 10 grams perliter of fluoride ion, and from about 0.1 gram to about 10 grams perliter of a prusside selected from the group consisting ofNa2(Fe(CN)5NH3); Na3(Fe(CN)5NH3); Na3(Fe(CN)5H2O); Na2(Fe(CN)5H2O) andNa3(Fe(CN)5NO2).
 9. The method according to claim 8 wherein the aqueoussolution is defined by claim 2.